Various solid, highly brominated aromatic organic compounds, i.e., compounds which contain at least about 60 wt % of total bromine, are useful, e.g., as flame retardants for thermoplastic and/or thermosetting polymeric resins, or for other flammable materials. Such compounds are usually formed by bromination of the unbrominated aromatic compound with excess bromine in the presence of a bromination catalyst and in a reaction mixture having a liquid phase which may be excess bromine itself. Unfortunately, many such highly brominated compounds formed in this manner, also have elemental bromine trapped within their particles or crystals. Such bromine is often referred to as occluded bromine. The presence of such bromine in the compound is undesirable because the entrapped bromine imparts color to the particles, and can lower the thermal stability of the flame retardant and of the polymer in which the flame retardant is used. Also during thermal processing of thermoplastic polymers containing such highly brominated compounds as flame retardants, bromine and/or HBr can be released and these substances are highly corrosive to materials used in the fabrication of such processing equipment.
Because these highly brominated aromatic compounds have poor solubility in common organic solvents, it is not economically feasible to purify the compounds by typical washing or recrystallization procedures. Too much solvent would be required.
U.S. Pat. Nos. 6,518,468; 6,603,049; 6,768,033; 6,958,423; and 6,974,887 describe, inter alia, commercially-used process technology for effectively reducing the occluded bromine content of decabromodiphenylethane product, one of the commercially important members of the highly brominated aromatic organic compounds with which this invention is concerned. That process technology involves oven-aging at elevated temperatures, previously ground and dried product.
U.S. Pat. No. 5,457,248 describes as one of its features, a high temperature aromatic solvent treatment of a brominated diphenylalkane to improve its color characteristics. It is pointed out that the aromatic solvent used in such high temperature treatment must not decompose at the temperatures at which the brominated diphenylalkane dissolves in the solvent. Such temperatures are stated to be, typically, in excess of 175° C. and often in excess of 200° C., e.g., usually about 200° C. to about 300° C. To perform such high temperature treatment, the brominated diphenylalkane is contacted with the aromatic solvent at a temperature sufficiently high that the brominated diphenylalkane is dissolved in the solvent, preferably forming a solution essentially free of solid diphenylalkane material. Preferred aromatic solvents are indicated to be non-fused aromatic compounds such as biphenyls, diphenylalkanes, (e.g. with C1 to C10 alkylene bridges), diphenyloxides, diphenylsufones, diphenylcarbonates, and the like, as well as fused aromatic solvents such as naphthalene or naphthalene derivatives such as alkyl naphthalenes. After complete dissolution of the brominated diphenylalkane, the solution is caused to be at a temperature at which the brominated diphenylalkane precipitates. Although it is stated that high product recoveries can be readily achieved, the primary figure given is 75% or more. Although recoveries ranging even up to about 90 to 95 wt % or more are mentioned, it appears that at least some loss of product is to be expected by use of the solvent process of this patent. Also, because many of the aromatic solvents, including the preferred diphenylalkanes and diphenyloxide, are solids at room temperature, special procedures are recommended to effect separation between the brominated diphenylalkanes and the aromatic solvent. One such procedure includes hot filtration to avoid solidification of the solvent itself along with the brominated product. Another such procedure includes allowing both the aromatic solvent and the brominated diphenylalkane to solidify. The resultant solid mixture can then be treated with another liquid organic solvent in which the solid solvent material, but not the brominated diphenylalkane product, is soluble. The brominated diphenylalkane product can then be filtered from the resulting medium. In conducting the high temperature aromatic solvent procedure of the patent, the special hot filtration procedure was used and in each such case the filtrate resulting from the high temperature filtration was washed with an ordinary organic solvent (methanol or acetone), the filtrate was reslurried in the same solvent and filtered, and this filtrate was washed with the same solvent. Obviously such special procedures add to the complexity and costs of the process.
Thus, procedures used heretofore for reducing occluded bromine content involved roasting the compounds at high temperatures or dissolving the compounds in a solvent at high temperature and then precipitating the compounds out of solution at lower temperatures. It would be of advantage if less complex and less costly procedures for achieving reductions in occluded bromine content could be found.